Method for the continuous vapor phase acetylation of cellulose fiber



United States Patent 3,320,022 METHOD FOR THE CONTBN'UOUS VAPOR PHASE ACETYLATION F CELLULSE FIBER Frederick J. Hill, Wallingford, Pa., assignor to FMC Corporation, Philadelphia, Pa., a corporation of Delaware Filed Apr. 27, 1964, Ser. No. 362,770 3 Claims. (Cl. 8-121) This invention relates to a method and means for the continuous vapor phase acetylation of cellulose liber.

The vapor phase acetylation of cellulose fibers presents many problems. A continuous process for the acetylation of rayon in the gaseous phase is even more difficult. Some of the problems to be overcome include isolation of the acetylating zone from moisture, prevention of leakage of acetylating vapor to surrounding atmosphere, temperature control of exothermic reaction and economic use and recovery of acetylating vapors.

Some of these problems have been dealt with successfully in the prior art, for example see U.S. Patent No. 3,045,461, however, it is a primary object of this invention to provide a more economical and more easily controlled continuous process for the vapor phase acetylation of cellulose fibers.

It is a further object of this invention to provide a continuous process for the vapor phase acetylation of cellulose fibers having increased chemical eiiciency and a faster acetylation rate.

It is a further object of this invention to provide an apparatus for the continuous vapor phase acetylation of cellulose fibers which has va greater capacity and reduced size.

It is a further object of this invention to provide an apparatus for the continuous vapor phase acetylation of cellulose fibers which will facilitate control of the process and permit more economical use of the acetylating vapors.

These and other objects are accomplished in accordance with this invention in a process for the continuous vapor phase acetylation of a catalyst impregnated cellulose liber wherein the fiber is continuously moved through an acetylating zone on a foraminous conveying means, the improvement which comprises feeding an acetylating gas into the acetylating Zone adjacent the liber exit end and in a countercurrent direction to the movement of the liber through said zone, said acetylating gas consisting essentially of from 75 to 95% by volume of acetic anhydride and from 25 to 5% by volume of acetic acid vapor, and removing the spent acetylating gas adjacent the fiber inlet to the acetylating zone. In a more specific embodiment the acetylating gas is caused to flow in a substantially perpendicular path through the cellulose liber while it is generally moving in a countercurrent direction thereto. Thus, the gas is moved essentially from the last stage to the first stage in a circuitous manner traveling perpendicularly through the conveyed fiber in each stage of acetylation.

In the preferred embodiment of this invention the liber s conveyed through the acetylating zone at the rate of about 0.75 to 1.8 pounds per square foot of conveyor surface and the acetylating gas is fed through the acetylating Zone at a velocity of from about 70 to 160 cubic feet per minute per square foot of conveyed fiber. The fiber should be compressed as it travels through the acetylating zone to a bulk density of from 1.3 to 2.0 pounds per cubic foot.

The process of the invention may be accomplished with the use of an apparatus comprising a multi-stage acetylating zone chamber, each of said stages comprisingy adjacent, closed housings having fiber conveying inlets and outlets, means to convey cellulose fiber through said multi-stage acetylating zone, said means preferably cornprising at least one foraminous endless belt, gas supply means connecting each adjacent stage, said gas .supply means having inlets and outlets on opposite sides of the conveying means whereby gas flow will be through the liber on said conveying means, acetylating vapor supply means for the last acetylating stage through which the fiber conveying means travels, and gas exhaust means from the first of said acetylating stages through which the conveying means travels.

The invention will be more readily understood when considered in connection with the accompanying drawing wherein FIG. I is a schematic, cross-sectional View of the apparatus of this invention; and

FIG. II is a more detailed cross-sectional view of the apparatus as shown along line I-I of FIG. I.

In the drawing, continuous, perforated or screen endless belts 2 and 4 come together to convey cellulose fiber 6 through the acetylator generally designated as 8. The liber 6 is conveyed through a primary moisture removing and vapor escape prevention zone 10, and then through acetylating zone stages 12, 14 and 16, respectively. After the last stage 16, the liber is conveyed through another vapor escape prevention zone 18 and then out of the acetylator for further processing.

Line 20 carries acetylating vapor which is a mixture of pure acetic anhydride vapor from vaporizer 22 and a mixture of acetic anhydride vapor and acetic acid vapor from fractionating condenser 24. The mixture of gases proceeds to the last acetylating stage 16 where it flows through partially acetylated liber 6 on conveyors 2 and 4. T he partly spent acetylation mixture is then drawn through line 26 into stage 14 where it reacts with the partially acetylated fiber coming from stage 12 by passing through the perforated conveyor belts 2 and 4. From stage 14, the further spent acetylating mixture is drawn into stage 12 through line 28 and then through the unreacted liber 6 entering stage 12 of the acetylator 8. Spent acetylating vapors `are passed through line 30 into the fractionating condenser 24 for recovery of acetic anhydride.

The advantage of this countercurrent process is that a rich acetylating mixture is brought in contact with the partially acetylated fibers and speeds up the acetylation process Without causing excessive heat to be generated in the exothermic reaction while subsequent treatments of the lesser and non-acetylated fiber take place with more dilute acetic anhydride vapors. This is important since the initial reaction of unreacted iiber with an acetylating vapor is quite rapid and highly exothermic requiring a more dilute acetylating mixture to prevent burning or charring of the fibers. As the acetylation proceeds the reaction slows and requires a greater concentration of acetic anhydride to hasten acetylation of the bers to the desired degree.

In FIG. II, the cross-section of stage 14 of the acetylator 8 is shown in more detail. Insulated sides 32 are necessary to prevent condensation and heat loss While pump 34 is used to obtain better control of temperature and gas flow. Other pumps may be employed for each stage of the acetylator, and heat transfer means (not shown) may be employed to remove heat from the acetylating vapors during travel from one stage to another.

The described apparatus permits faster acetylation of the cellulose fiber thus allowing for a shorter reaction zone and generally shorter equipment.

The following example is -set forth to show the method of this invention in an apparatus as previously described.

Example Dry rayon staple ber impregnated with 15% potassium acetate based on the Weight of the ber is fed to the acetylator 8 between perforated conveyor belts 2 and 4 at the rate of 0.75 pound per square foot of conveyor surface. Belts 2 and 4 compress the `staple to a bulk density of 1.7 pounds per cubic foot, and convey the staple fiber through zone which is effectively sealed and purged with a dry inert gas to exclude atmospheric air and to prevent leakage of acetylation vapor into the surrounding atmosphere.

A feed vapor is introduced to stage 16 of the acetylator through line 20 at a velocity of 90 cubic feet per minute per square foot of conveyed fiber. This rate is necessary under these reaction conditions to minimize thermal damage to the fiber by the exothermic heat of reaction and to obtain a uniform degree of acetylation through the staple blanket. The feed vapor is a. mixture of about 85% by volume of acetic anhydride vapor and 15% by volume of acetic acid vapor. The temperature of the feed vapor as it proceeds in a countercurrent direction through the acetylation stages is maintained at about 10 F. above the vapor dewpoint of the vapor mixture. Vapor pumps as seen in FIG. II at 34 are used to regulate the countercurrent ow of acetylation vapor through the stages. The feed vapor is continually diluted with vaporous acetic acid which is a product of the reactionand which cuts down the activity of the feed vapor as it proceeds through the acetylation zone, Finally the spent acetylating vapors at a ratio of about 50 parts acetic anhydride vapor and 50 parts of acetic acid vapor is pulled out of stage 12 and back to a fractionating condenser which separates excess acetic acid and recycles an acetic anhydride rich mixture into the feed gas line for return to the acetylator.

The staple ber is conveyed out of the acetylator through purge and seal zone 18 and on to further processing. The time in the acetylating Zone is about 15 minutes to produce a cellulose acetate fiber having an acetyl content of 35% Various changes and modifications may be made practicing the invention without departing from the spirit and scope thereof and, therefore, the invention is not to be limited except as defined in the appended claims.

I claim:

1. In a process for the continuous vapor phase acetylation of diy, catalyst impregnated cellulose fiber wherein the ber is continuously moved on a conveying means through an acetylating Zone having a ber inlet and outlet, the improvement which comprises feeding into the portion of said acetylating zone adjacent said fiber outlet, an acetylating vapor initially consisting of a mixture of 75 to 95% by volume of acetic anhydride vapor and from 25 to 5% by volume of acetic acid vapor, acetylating the moving ber with said acetylating vapor moving in a direction generally countercurrent to the movement of the ber and the acetylating vapor becomes progressively more spent and dilute in acetic anhydride as it moves toward the fiber inlet portion of said acetylating zone, and removing the spent acetylating vapor adjacent said ber inlet.

2. The process of claim 1 wherein the acetylating vapor is also caused to flow substantially perpendicular through the cellulose fiber as it passes through the acetylating zone.

3. The process of claim 2 wherein the cellulose ber is conveyed through the acetylating zone at the rate of from 0.75 to 1.8 pounds per square foot of conveyor surface and the acetylating vapor is fed through the acetylating zone at a velocity of from about to 160 cubic feet per minute per square foot of conveyed fiber.

References Cited by the Examiner UNITED STATES PATENTS 3,045,461 7/1962 Saito 8--121 X FOREIGN PATENTS 577,103 5/ 1946 Great Britain. 930,208 7/1963 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

H. WOLMAN, Assistant Examiner. 

1. IN A PROCESS FOR THE CONTINUOUS VAPOR PHASE ACETYLATION OF DRY, CATALYST IMPREGNATED CELLULOSE FIBER WHEREIN THE FIBER IS CONTINUOUSLY MOVED ON A CONVEYING MEANS THROUGH AN ACETYLATING ZONE HAVING A FIBER INLET AND OUTLET, THE IMPROVEMENT WHICH COMPRISES FEEDING INTO THE PORTION OF SAID ACETYLATING ZONE ADJACENT SAID FIBER OUTLET, AN ACETYLATING VAPOR INITIALLY CONSISTING OF A MIXTURE OF 75 TO 95% BY VOLUME OF ACETIC ANHYDRIDE VAPOR AND FROM 25 TO 5% BY VOLUME OF ACETIC ACID VAPOR, ACETYLATING THE MOVING FIBER WITH SAID ACETYLATING VAPOR MOVING IN A DIRECTION GENERALLY COUNTERACTING TO THE MOVEMENT OF THE FIBER AND THE ACETYLATING VAPOR BECOMES PROGRESSIVELY MORE SPENT AND DILUTE IN ACETIC ANHYDRIDE AS IT MOVES TOWARD THE FIBER INLET PORTION OF SAID ACETYLATING ZONE, AND REMOVING THE SPENT ACETYLATING VAPOR ADJACENT SAID FIBER INLET. 